Carbon filaments capable of substantial crack diversion during fracture

ABSTRACT

An improved carbon filament is provided having an internal structure capable of increasing the amount of work required to break the filament. The internal structure of the carbon filaments of the present invention as evidenced by the means apparent fracture surface energy of the same facilitates the exhibition of satisfactory strength properties even if accompanied by the presence of gross inhomogenities and structural flows such as commonly encountered in carbon filaments of the prior art.

United States Patent Ram et al.-

[ CARBON FILAMENTS CAPABLE OF SUBSTANTIAL CRACK DIVERSION DURINGFRACTU'RE Inventors: Michael J. Ram, West Orange; John P. Riggs, BerkleyHeights, both of NJ.

Celanese Corporation, New York, NY.

Filed: Apr. 17, 1972 Appl. No.: 244,544

Related US. Application Data Continuation-impart of Ser. No. 28,545,April 14, 1970, Pat. No. 3,657,409.

Assignee:

US. Cl. 57/140 R, 423/447 Int. Cl. D02g 3/02 Field of Search 57/140 R,140 BY, 139; 423/447; 8/1 15.5

[56] References Cited UNITED STATES PATENTS 3,532,466 10/1970 Johnson eta1. 423/447 Oct. '15, 1974 Prescott ct a1 423/447 Holsten et a1 423/447Townscnd..... 423/447 Joo et a1. 423/447 Douglas ct 423/447 Stuetz cta1. 423/447 Ezekiel 423/447 Ezekiel 423/447 Primary Examiner-.lohnPetrakes ABSTRACT An improved carbon filament is provided having aninternal structure capable of increasing the amount of work required tobreak the filament. The internal structure of the carbon filaments ofthe present invention as evidenced by the means apparent fracturesurface energy of the same facilitates the exhibition of satisfactorystrength properties even if accompanied by the presence of grossinhomogenities and structural flows such as commonly encountered inCarbon filaments of the prior art.

10 Claims, 4 Drawing Figures RATENTEDUBT] 51914 SHEET 10F 3 FIG.

FIG

MEAN smGLE HLAMENT TEN\LE STRENGTH (Kps) RAIENTEDUET 1 W $841079 lOOO80o AVERAGE FLAW SIZE,C

30o \S/XL/ 100 Z \0 '10 so 40 so I00 MEAN AFPARENT FRACTURE SURFACEENERGY /EEQUMZE METER) CARBON FILAMENTS CAPABLE OF SUBSTANTIAL CRACKDIVERSION DURING FRACTURE CROSSREFERENCE TO RELATED APPLICATION This isa continuation-in-part of our U.S. Ser. No. 28,545, filed Apr. 14, 1970entitled Improved Process for the Production of Acrylic Filaments" (nowU.S. Pat. No. 3,657,409).

' BACKGROUND OF THE INVENTION Carbon filaments have long been known andare discussed in the technical literature. It has generally beenrecognized that the structure of the carbon filaments is influenced tosome degree by the nature of the fibrous material which is thermallyconverted irito the carbon filaments and by the processing conditionsutilized during the thermal conversion. It has also been recognized thatcarbon filaments are known to possess an internal structure which issomewhat fibrillar in nature and that some micropores (i.e., microvoids)in addition to the usual structural flaws may be detected within thesame.

See, for instance, the article by R. Perret and W. Ruland appearing inJ. Appl. Cryst., Vol. 3, Pages 525-532 (1970), entitled TheMicrostructure of PAN-Base Carbon Fibres.

In the search for high performance materials considerable interest hasbeen focused upon carbon fibers. Industrial high performance materialsof the future are projected to make substantial utilization of fiberreinforced composites, and carbon fibers theoretically have among thebest properties of any fiber for use as a high strength reinforcement.Among these desirable properties are corrosion and high temperatureresistance, low density, high modulus and high tensile strength. Carbonfiber reinforced composites are commonly formed by incorporating carbonfilaments in a resinous or metallic matrix. Representative uses forcarbon fiber reinforced composites include aerospace structuralcomponents, rocket motor casings, deep-submergence vessels and ablativematerials for heat shields on reentry vehicles, etc.

Heretofore, those material scientists interested in attempting toimprove the internal structure of carbon filaments have directed theirattention largely to elimination of strength reducing flaws within thesame. See, for instance, the article by J.W. Johnson and DJ. Thorneappearing in Carbon, Vol. 7, Pages 659461 (1969), entitled Effect ofInternal Polymer Flaws on Strength of Carbon Fibres Prepared From anAcrylic Precursor, and the article by John W. Johnson appearing inApplied Polymer Symposia, Vol 9, Pages 229-243 (1969), entitled FactorsAffecting the Tensile Strength of Carbon-Graphite Fibres."

The present invention provides a novel approach to the improvement ofcarbon filaments. The carbon filaments of the present invention possessan internal structure unlike that exhibited by the carbon filaments ofthe prior art as discussed in detail hereafter.

It is an object of the invention to provide improved carbon filaments.

It is an object of the invention to provide carbon filaments possessingan improved internal structure.

It is an object of the invention to provide carbon filaments capable ofsubstantial crack diversion upon fracture.

It is an object of the invention to provide an improved carbon filamentpossessing an unusually highly developed microporous and fibrillarinternal structure.

It is another object of the invention to provide an improved carbonfilament possessing an internal structure which facilitates exhibitionof highly satisfactory strength properties even if accompanied by thepresence of structural flaws such as commonly encountered in carbonfilaments of the prior art.

These and other objects as well as the scope, nature, and utilization ofthe invention will be apparent from the following description andappended claims.

SUMMARY OF THE INVENTION An improved carbon filament is providedcomprising at least percent carbon by weight having an unusually highlydeveloped microporous and fibrillar internal structure substantiallycoextensive with the length of the filament capable of diverting apropagating crack during fracture thereby increasing the amount of workrequired to break the filament as evidenced by a mean apparent fracturesurface energy of at least 50 joules per square meter.

In a preferred embodiment of the invention the carbon filaments containat least percent carbon by weight and additionally exhibit a mean singlefilament Youngs modulus of about 20 to 50 million psi.

DESCRIPTION OF DRAWINGS FIG. 1 and FIG. 2 are photographs made with theaid of a scanning electron microscope at a magnification of 5,740X ofmatching sides of the primary fracture surface of a carbon filament ofthe present invention formed in accordance with the procedure describedin the Example which shows a flaw having a maximum dimension of 1.4micron.

FIG. 3 is a graph which illustrates for carbon filaments of 35 millionpsi Youngs modulus having various average flaw sizes the relationshipbetween the mean apparent fracture surface energy and the mean singlefilament tensile strength.

FIG. 4 is a schematic view of a representative apparatus arrangementsuitable for forming acrylic filaments which are subsequently thermallytransformed into the carbon filaments of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The improved carbon filaments ofthe present invention are derived from polymeric fibrous materials. Forinstance, an acrylic polymer may be formed into an acrylic filamentpossessing a requisite internal structure, and the resulting acrylicfilament converted to a carbon filament possessing an improved internalstructure through appropriate thermal processing.

Acrylic filament precursors suitable for thermal conversion into theimproved carbon filaments of the present invention may be formed inaccordance with embodiments of the process described in our commonlyassigned U.S. Ser. No. 28,545, filedl Apr. 14, 1970, and entitledImproved Process for the Production of Acrylic Filaments (now U.S. Pat.No. 3,657,409) which is herein incorporated by reference. Such improvedacrylic filaments are claimed in our commonly assigned U.S. Ser. No.244,54l, filed concurrently herewith, entitled Improved AcrylicFilaments Which Are Particularly Suited for Thermal Conversion to CarbonFilaments.

More specifically, the fiber-forming acrylic polymer selected for use inthe formation of the acrylic precursor may be either an acrylonitrilehomopolymer or an acrylonitrile copolymer which contains at least about85 mol percent of acrylonitrile units and up to about 15 mol percent ofone or more monovinyl units copolymerized therewith. An acrylonitrilehomopolymer is particularly preferred. Suitable copolymers commonlycontain at least about 95 mol percent of recurring acrylonitrile unitsand up to about 5 mol percent of one or more monovinyl unitscopolymerized therewith. Representative monovinyl units which may beincorporated in the acrylonitrile copolymers include styrene, methylacrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidenechloride, vinyl pyridine, and the like. The acrylic polymers may beformed by standard polymerization processes which are well known in theart. Minor quantities of preoxidation or graphitization catalysts mayoptionally be incorporated in the bulk acrylic polymer prior tospinning.

The solvent utilized to form the spinning solution may bedimethylacetamide. The solvent is sometimes identified asN,N-dimethylacetamide or DMAC, and the chemical formula CH CON(CH Thestandard technical or commercial grade of dimethylacetamide may beemployed as the solvent in the formation of the spinning solution.

The spinning solution may be prepared by dissolving sufficient acrylicpolymer in the dimethylacetamide solvent to yield a solution suitablefor extrusion containing from about to 30 percent arcylic polymer byweight based upon the total weight of the solution, and preferably fromabout l8 to 25 percent by weight. In a particularly preferred embodimentof the invention the spinning solution contains the acrylic polymer in aconcentration of about to 22 percent by weight based upon the totalweight of the solution. The low shear viscosity of the spinning solutionshould be within the range of about 80 to 3,000 poise measured at 25C.and preferably within the range of about 125 to 1,500 poise measured at25C. If the spinning solution low shear viscosity is much below about 80poise measured at 25C., spinning breakdowns commonly occur. If thespinning solution low shear viscosity is much above about 3,000 poisemeasured at 25C., extremely high spinning pressures are required andplugging of the extrusion orifice may occur.

In a preferred precursor formation technique the spinning solutionadditionally contains about 0.1 to 5.0 percent by weight based upon thetotal weight of the solution, and preferably about 0.5 to 2 percent byweight based upon the total weight of the solution of lithium chloridedissolved therein. The incorporation of lithium chloride serves thefunction of lowering and preserving upon standing the viscosity of thespinning solution. The desired solution fluidity and mobility areaccordingly efficiently maintained even upon the passage of time. Forinstance, it has been found that a solution comprising 22 parts byweight acrylonitrile homopolymer, 2 parts by weight lithium chloride,and 76 parts by weight dimethylacetamide solvent commonly exhibits arelatively constant low shear viscosity of about 150 poise measured at25C. after standing for 250 hours. A solution containing an even lesserconcentration of acrylonitrile homopolymer and no lithium chloride(i.e., 20 parts by weight polymer, and parts by weightdimethylacetamide) tends to increase in viscosity upon standing andexhibits a low shear viscosity of about 1,000 poise measured at 25C.after about 2 k hours. The lithium chloride may be dissolved in thedimethylacetamide solvent either simultaneously with the acrylic polymeror before or after the acrylic polymer is dissolved therein. Minorquantities of preoxidation or graphitization catalysts may optionally beincorporated in the spinning solution.

The spinning solution is preferably filtered, such as by passage througha plate and frame press provided with an appropriate filtration medium,prior to wet spinning in order to assure the removal of any extraneoussolid matter which could possibly obstruct the extrusion orifice duringthe spinning operation.

The spinning solution containing the fiber forming acrylic polymerdissolved therein is extruded into a coagulation bath under conditionscapable of forming an acrylic filament having an internal structurewhich is capable upon subsequent thermal treatment of yielding theimproved carbon filaments of the present invention.

It has been found that acrylic filaments possessing the requisiteinternal structure are produced when an essentially non-aqueouscoagulation bath is utilized having a temperature of about 0 to 45C.(preferably about 10 to 35C.) which consists essentially of about 55 topercent by weight of ethylene glycol and about 15 to 45 percent byweight of dimethylacetamide. When employing dimethylacetamide in thecoagulation bath in concentrations much greater than about 45 percent byweight, then filament breakage tends to occur at the spinneret. Whenemploying dimethylacetamide in the coagulation bath in concentrationsmuch less than about 15 percent by weight, then the resulting filamentstend to lose their substantially round crosssection and have a tendencyto exhibit a more pronounced bean-shaped configuration. In a preferredembodiment of the invention the coagulation bath consists essentially ofabout 60 to 75 percent by weight of ethylene glycol and about 25 to 40percent by weight of dimethylacetamide. In a particularly preferredembodiment of the invention the coagulation bath consists essentially ofabout 60 percent by weight of ethylene glycol and about 40 percent byweight of dimethylacetamide. At the relatively low coagulation bathtemperatures employed the coagulation rate tends to be relatively slowand to enhance the formation of the desired fiber internal structure.

The temperature of the spinning solution at the time of its extrusionshould be within the range of about 10C. to about C., and preferably atabout 20 to 30C. In a particularly preferred embodiment of the inventionthe spinning solution is provided at room temperature, e.g., about 25C.,which thereby facilitates expeditious handling and storage of the same.

The spinneret utilized during the extrusion may contain a single holethrough which a single filament is extruded, and preferably contains aplurality of holes whereby a plurality of filaments may besimultaneously extruded in yarn or tow form. For instance, tows of up to20,000, or more, continuous filaments may be formed. The spinneretpreferably contains holes having a diameter between about 50 to micronswhen producing relatively low denier filaments having an asspun denierof about 8 to 24 denier per filament, and

holes of about 300 to 500 microns when producing relatively high denierfilaments having as as-spun denier of about 100 to 1,500 denier perfilament. Extrusion pressures between about 100 and 700 psig may beconveniently selected, and preferably between about 100 and 400 psig.Spinning or extrusion speeds of about 0.5 to meters per minute (e.g., 3to 6 meters per minute) may be employed.

Throughout the extrusion process the coagulation bath is preferablycirculated. A relatively constant composition within the coagulationbath may be maintained through the continuous withdrawal andpurification of the same. Alternatively, additional ethylene gly col maybe continuously added to the coagulation bath to preserve the desiredproportion of dimethylac- V etamide to ethylene glycol within the same.The length of the coagulation bath is adjusted so that the resultingas-spun filaments are present within the coagulation bath for aresidence time of at least about 6 seconds.

For instance, residence times of about 6 to 300 seconds may beconveniently selected. Residence times less than about 6 seconds tend toresult in an insufficiently developed fibrillar structure within theas-spun filaments. Residence times for the as-spun filaments in thecoagulation bath in excess of 300 seconds tend to yield no commensurateadvantage. Particularly preferred residence times for the as-spunfilament in the coagulation bath range from about 6 to 50 seconds.

The resulting as-spun filament is next washed with water to removedimethylacetamide solvent from the same. The as-spun filament ispreferably washed with water until substantially all residual amounts ofsolvent, coagulation bath, and inorganic compound (e.g., lithiumchloride), if any, are removed from the same. It is essential that thefilament first be exposed to a relatively cool water wash medium at atemperature of about 10 to 50C. and preferably at about 10 to 30C., andmost preferably at room temperature (e.g., about 25C.), for at leastabout 25 seconds. The entire wash treatment may be conducted at atemperature within the range of about 10 to 50C. Alternatively the washing of the filament may be subsequently continued at a more highlyelevated temperature, e.g., in excess of about 50C. to remove additionalsolvent. In a preferred embodiment of the invention the initial coldwater wash is conducted for at least about 50 seconds.

When the entire wash is conducted at a relatively cool wash temperatureof about 10 to 50C., wash times of about 25 to 240 seconds andpreferably about 50 to 120 seconds are commonly utilized depending uponthe filament denier. Longer wash times tend to yield no commensurateadvantage.

It has been found that the initial cool water wash described above isessential in order to preserve the requisite fiber homogeneity in theacrylic filament precursor. During the cool water wash a one-waytransfer of residual quantities of the dimethylacetamide spinningsolvent out of the filament is believed to be promoted to thesubstantial exclusion of the passage of the molecules of the water washmedium into the filament. It has been found that if the as-spun filamentis initially washed at a temperature substantially higher than about50C., then the resulting washed filament tends to contain a significantnumber of macrovoids and tends to flatten. At temperatures below about10C. the washing procedure tends to be unduly slow. The residualdimethylacetamide content of the washed acrylic filaments preferably isno more than about 5 percent dimethylacetamide by weight, and. mostpreferably no more than about 0.1 percent by weight, prior to subsequentprocessing.

The water wash treatment is conveniently conducted in an in-lineoperation with the filament after it leaves the coagulation bath beingcontinuously passed through a water wash medium which is continuouslyregenerated. Conventional filament wash rolls may be utilized. Thefilament alternatively may be washed with water while wound upon aperforated bobbin, or by the use of other washing means as will beapparent to those skilled in the art.

The as-spun and washed acrylic filament is drawn or stretched from about1.5 times its original length up to the point at which the filamentbreaks to orient the same and to thereby enhance its tensile properties.

Total drawratios above about 1.521 to 15:1 may commonly be selected. Thedrawing is commonly con ducted at an elevated temperature: andpreferably at a total draw ratio of between about 3:1 and 12:1. Thedense internal filament structure makes possible the use of therelatively high total draw ratios indicated. As will be apparent tothose skilled in the art, the drawing of the as-spun and washed acrylicfilament may be conducted by a variety of techniques. For instance, itis possible for the drawing to be conducted while the filament is (a)immersed in a heated liquid draw medium, (b) suspended in a heatedgaseous atmosphere, (e.g., at a temperature of about 120 to 200C.) or(c) in contact with a heated solid surface (e.g., at a temperature ofabout 130 to 170C). If desired, the total draw imparted to the filamentmay be conducted by a combination of the foregoing techniques. When drawtechniques (b) and (c) are utilized, it is essential that the acrylicfilament be provided to the draw zone in an essentially dry form inorder to avoid formation. When draw technique (a) is employed, theacrylic filament is subsequently washed to remove the draw medium and isdried. Additionally, the liquid draw medium may also serve a washingand/or coagulating function wherein residual quantities ofdimethylacetamide are removed from the water washed fiber.

In a preferred embodiment of the invention the washed acrylic filamentis at least partially drawn while immersed in a hot glycerin bath. In aparticularly preferred embodiment of the invention the filament is drawnwhile immersed in a hot glycerin bath at a temperature of about to 110C. and at a draw ratio of about 1.511 to 3:1 (preferably at atemperature of about C. and a draw ratio of about 2:1), washed in coolwater (e.g., at a temperature of about 10 to 50C.), and subsequentlydrawn at a draw ratio of about 3:1 to 6:1 while in contact with a hotshoe at a temperature of about to 220C, and preferably at a temperatureof about to C.

The drawn acrylic filaments optionally may be plied to form yarns ortows of increased total denier as will be apparent to those skilled inthe art prior to thermal conversion into the improved carbon filamentsof the present invention as described hereafter.

The resulting acrylic filaments are converted to a stabilized orheat-resistant form which is capable of undergoing carbonization. Thestabilization treatment renders the acrylic filaments non-burning whensubjected to an ordinary match flame while retaining their originalfibrous configuration essentially intact. The stabilization reaction maybe conducted by heating the acrylic filaments at moderate temperaturesin accordance with techniques known in the art. Such a stabilizationprocedure is commonly conducted in the presence of oxygen and results inthe formation of a cyclized and preoxidized product which exhibits athermal stability not exhibited by the unmodified acrylic filaments.While it is possible that the stabilization reaction be conducted on abatch basis, it is preferable that the stabilization reaction beconducted on a continuous basis. Catalyzed stabilization reactionsoptionally may be selected. The exact stabilization temperaturesemployed will vary with the chemical composition of the acrylicfilaments. Preferred stabilization procedures are described in commonlyassigned U.S. Ser. No. 749,957, filed Aug. 5, 1968 (now abandoned), ofDagobert E. Stuetz, and in US. Pat. No. 3,539,295, of Michael J. Ram,which are herein incorporated by reference. Other stabilizationprocedures capable of imparting thermal stability to the acrylicfilaments may be selected. The highly fibrillar internal structurerequired to make possible the formation of the claimed carbon filamentsis retained throughout the stabilization reaction.

The stabilized acrylic filaments are converted to the carbon filamentsof the present invention by thermal treatment at a more highly elevatedtemperature of at least l,000C., e.g. l,000 to 2,000C. in a nonoxidizingatmosphere. Preferably inert atmospheres such as nitrogen, argon andhelium are employed. The stabilized acrylic filaments are subjected tosuch highly elevated thermal treatment until carbon filaments containingat least 90 percent carbon by weight are formed, and preferably untilcarbon filaments containing at least about 95 percent carbon by weightare formed. In a more particularly preferred embodiment carbon filamentscontaining at least 98 percent carbon are formed. Carbon filaments ofoptimum tensile strength are formed when the maximum temperatureprovided in the heating zone is about 1,500 to l,900C. (e.g., 1,800C.).The carbon fibers are preferably formed on a continuous basis bycontinuous passage through a heating zone containing a non-oxidizingatmosphere and a temperature gradient in which the stabilized acrylicfilaments are gradually raised to the maximum carbonization temperature.During the thermal treatment at l,000 to 2,000C. a highly developedmicroporous structure is inherently imparted to the resulting carbonfilaments wherein a large number of elongated micropores of up to about25 Angstroms (e.g., about to Angstroms) in thickness are disposedbetween the highly fibrillar internal structure which are largelypreserved during the thermal treatment. The increased presence of themicropores within the highly fibrillar internal structure is confirmedby small angle X-ray analysis.

The carbon filaments of the present invention commonly exhibit a meansingle filament Youngs modulus of about to 50 million psi. As discussedhereafter, the internal structure of the carbon filaments of the presentinvention provides carbon filaments of im proved tenacity in spite ofstructural flaws such as those commonly encountered in the prior art.For instance, carbon filaments commonly possess flaws of 0.5 micron andlarger as discussed in the Johnson, and Theme and Johnson articles citedearlier.

The improved carbon fibers of the present invention have an unusuallyhighly developed microporous and fibrillar internal structure which iscapable of diverting a propagating crack during fracture therebyincreasing the amount of work required to break the filament asevidenced by a mean apparent fracture surface energy (i.e., 7a) of atleast 50 joules per square meter (e.g., 50 to 150, or more, joules persquare meter). In a preferred embodiment the carbon filaments exhibit amean apparent fracture surface energy of at least 60 joules per squaremeter. In a particularly preferred embodiment the carbon filamentsexhibit'a mean apparent fracture surface energy of at least joules persquare meter.

It has been found that carbon/graphite filaments fail in a brittlefashion and that the Griffith failure criterion can be utilized toelucidate the fracture phenomenology. One form of the simple Griffithequation which has been applied to the fracture of many brittlematerials is as follows:

WTB /2 L 6 Lu /2E 'Ya/7 C where WTB work-to-break (energy to fracture).

L breaking stress 6,, breaking strain E Young's modulus y apparentfracture surface energy C size of critical flaw at which fractureinitiated For an ideally brittle material, y should correspond to theenergy required to break primary chemical bonds. It is recognized,however, that in all but extremely rare cases of apparently brittlefracture, large amounts of plastic work are done at the crack tip,leading to observed values of y much higher than would be expected frombond-breaking alone. See, for example, Fracture edited by H. Liebowitz,Academic Press, New York (1968). The apparent fracture surface energy, yis an intrinsic property of the carbon fiber and accordingly isdependent upon the internal physical structure of the carbon fiber.

A technique for the determination of the mean apparent fracture surfaceenergy, y for a given carbon filament is described in detail below.Generally stated the WTB and C for a given carbon filament aredetermined and the mean apparent fracture surface energy is calculatedtherefrom.

1. Single carbon filaments individually are broken while immersed inglycerin and the stress, strain, and Youngs modulus determined byconventional fiber testing techniques. The use of a glycerin bathminimizes the formation of secondary fracture surfaces which tend to beformed in an open atmospherev The resulting pair of ends for each brokenfilament is examined in the field of a scanning electron microscope andcompared to assure a match thus insuring that a primary fracture surfaceis being examined. See, for instance, FIG. 1 and FIG. 2 which arephotographs made with the aid of a scanning electron microscope of theprimary fracture surface of a carbon filament of the present inventionformed in accordance with the procedure described in the Example. 3. Thecritical flaw, C, which initiated the fracture is located and thelongest dimension on the primary fracture surface is measured. In FIG. 1and FIG. 2

the largest dimens o Of th flaw On the prim y The following example isgiven as a specific illustrafracture Surface o each brOken end was foundI tion of the invention with reference being made to FIG. be 1.4 m c on.4 of the drawings. it should be understood, however. The above Procedureis repeated 3 number of times that it is not essential that the improvedcarbon filafor a given Sample of Carbon filaments and the Valufis 5ments of the present invention be formed through the obtained for i l/TBand C are plot e as log WTB 8- log utilization of the exact processingparameters set forth C. A least squares line is drawn through the pointsasi h E l suming a slope of l in accordance with the theoreticalprediction of the Griffith equation. The intercept of this line isaccordingly log ('y,,/1r) from the which the 10 EXAMPLE mea y, or meanapparent fracture surface energy, is 1 determined. It is recommendedthat at least four car- Twentytwo parts b i h f l l i il h bon filamentsbe broken as described above when del r, 2 parts by weight of lithiumchloride, and

termining the mean apparent fracture surface energy 76 rt by weight ofindustrial grade dimethylacfor a given sample of carbon filaments. Theresults obetamide are slurried at room temperature for 120 mintainedare, of course, more statistically accurate as the Utes b use of astirred vessel. The slurry is heated to number of breaks increases. atemperature of 100C. over a period of about 90 min- The mean apparentfracture surface energies of a utes where it is mixed with agitation for2 hours. The wide variety of carbon filaments have been determinedresulting solution is passed four times through a conln all instances,the mean apparent fracture surface enventional filter press while at100C. over a period of ergies were substantially below that of thecarbon fila- 19 hours in order to remove any solid contamination. mentsof the present invention. The following Table I The low shear viscosity(Brookfield) of the resulting sets forth the average flaw size and meanapparent fracspinning solution after degassing is found to be about turcsurface energy obtained for representative carbon 120 poise measured at27C. filaments. The spinning solution is provided in dope bomb 1 TABLE 1Carbon Carbon Average Single Average Mean Apparent Fiber Source FiberDesignation Filament Young's Flaw Fracture Surface Modulus Size EnergyMpsi micron joules/sqm.

Great Lakes Carbon 37" 31 0.8 33 Corp. 4T 39 0.8 31 ST 48 0.9 37 6T 600.9 26 Hercules, lnc. l'lT-S 39 0.7 4] A5 36 1.1 42 HM-s 64 1.4 25Johnson & Thorne Acrilan 20 0.7 39

Carbon Fibers Cclanese A 1.1 33 B4 35 0.5 29 C 35 1.4 29 D 37 1.4 14

l Derived from a dry spun Orlon acrylic precursor.

2 Derived from a we! spun Courtelle acrylic precursor (sodiumthiocyanate spinning solution).

3 Shown in H6. 5. following Page 661. of Vol. 7. of Carbon. article byJ. W. Johnson and D. J. Thorne cited earlier (wet spun from DMACspinning solution employing an aqueous coagulation bath).

4 Derived from various dry spun acrylic precursors.

An unusually high mean apparent fracture surface under an atmosphere ofnitrogen at 20 psig. The spinenergy enables a larger average flaw sizeto be tolerated ning solution is conveyed to spinneret 2 via the line 4while still obtaining a high strength carbon filament. As where it isextruded into coagulation bath 6. The spinfracture is initiated and apropagating crack meets a neret 2 is of the standard cup type andcomprises a sinmicrovoid between fibrils, the crack is diverted andadgle circle of 400 holes each having a diameter of 100 ditional energyis consumed. For example, see the folmi r lowing Table ll. Thecoagulation bath consists of parts by weight TABLE ll Mean ApparentFracture Surface FIG 3 illustrates for carbon filaments of 35 millionethylene glycol and 40 parts by weight dimethylacpsi Youngs modulushaving various average flaw sizes etamide and is provided at atemperature of 36C. The the relationship between the mean apparentfracture coagulation bath is caused to flow concurrently with surfaceenergy and the mean single filament t il coagulated filament 8 and ismaintained at a relatively strength. constant composition by thecontinuous addition of ethylene glycol to the same and the continuouswithdrawal of a portion of the bath. The coagulation bath has a lengthof 37 inches and the coagulated filaments are maintained in the same fora residence time of about 9 seconds.

The coagulated filaments pass under guide 10 which is immersed incoagulation bath 6 and are conveyed to a skewed roll 12 and wash roll 14which is partially immersed in water bath 16 which is maintained at 23C.The coagulated filaments are taken up on roll 12 at a rate of 6 metersper minute. The filaments are wrapped about skewed roll 12 and wash roll1141 for a residence time of about 125 seconds during which time thefilaments are immersed in water for approximately 25 seconds andwithdrawn with water adhering to the same during which timesubstantially all residual amounts of dimethylacetamide are removed fromthe same.

The washed filaments are next continuously passed through stretch bath17 having a length of inches which is provided with glycerin at 80C.Rollers 18 and 20 situated outside the stretch bath and rollers 22 and24 immersed within the stretch bath guide the filaments during thestretching operation. The filaments are next taken up on skewed roll 26and wash roll 28 which is partially immersed in a water wash bath 30provided at 22C. which is substantially identical to that of wash bath16. The filaments are taken up on skewed roll 26 at a rate of 12 metersper minute and are accordingly drawn at a draw ratio of 2:1 whileimmersed in the glycerin stretch bath. The filaments are immersed in thewater wash bath 30 for approximately 12.5 seconds during which timeresidual quantities of glycerin are substantially removed from the same.The water present in wash bath 30 is circulated and is constantlyregenerated. The washed filaments are next passed to skewed roll 32 anddrying roll 34 where residual quantities of moisture are expelled fromthe same. Drying roll 34 is steam heated and maintained at a constanttemperature of approximately 95C.

The washed and dried filaments are next passed over a 2-foot heated drawshoe which is provided at a constant temperature of 145C. The residencetime of the filaments while in contact with the hot shoe 36 is 1.25second. The drawn filaments are collected on takeup roll 38 at a rate of60 meters per minute.

Two of the drawn filament bundles are next plied to form a continuouslength of acrylic fibrous material consisting of 800 continuousfilaments. This resulting continuous length is next subjected to a briefthermal pretreatment in accordance with the teachings of commonlyassigned U.S. Ser. No. 17,962, filed Mar. 9, 1970 (now abandoned). Morespecifically, the continuous length is passed continuously through anoven provided with an air atmosphere at 195C. for a residence time ofabout 240 seconds while maintaining the longitudinal tension thereon sothat 10.5 percent shrinkage in length takes place.

The continuous length of thermally pretreated acrylic fibrous materialisnext passed for about 180 minutes through a multiple roll ovenprovided with an air atmosphere at 266C. While passing through this oventhe acrylic fibrous material is thermally stabilized and is renderedblack and non-burning when subjected to an ordinary match flame. Theresulting stabilized fibrous material retains its original fibrousconfiguration essentially intact, and contains a bound oxygen of about10.1 percent by weight when subjected to the Unterzaucher analysis.

The continuous length of stabilized filaments is next converted to theimproved carbon filaments of the present invention by passage through anInductotherm induction furnace utilizing a 20 KW power source. Theinduction furnace comprises a water cooled copper coil and a hollowgraphite tube suspended within the coil having a length of 38 inches andan inner diameter of 0.75 inch through which the continuous length ofstabilized filaments is continuously passed. The copper coil whichencompasses a portion of the hollow graphite tube is positioned at alocation essentially equidistant from the respective ends of thegraphite tube. An inert atmosphere of nitrogen is maintained within theinduction furnace. Air is substantially excluded from the inductionfurnace by purging with nitrogen. The continuous length of stabilizedfilaments is passed through the induction furnace at a rate of about 3inches per minute. A longitudinal tension of 0.2 grams per denier isexerted upon the continuous length of fibrous material as it passesthrough the induction furnace. The fibrous material is at a temperatureof about 150C. as it enters the induction furnace and is raised to atemperature of 800C. in about 150 seconds, and from 800C. to 1,500C. inabout 200 seconds where it was maintained at 1,500C. i 25C. for about 48seconds.

The resulting carbon filaments contain in excess of 98 percent carbon byweight, and are found to possess a mean apparent fracture surface energyof 82 joules per square meter. The mean apparent fracture surface energywas calculated as heretofore described.

FIG. 1 and FIG. 2 are photographs made with the aid of a scanningelectron microscope at a magnification of 5,740X of matching sides ofthe primary fracture surface of a representative carbon filament whichshows a flaw having a maximum dimension of 1 .4 micron. The average flawsize for the carbon filaments produced is found to be 2.7 microns.

The resulting carbon filaments additionally exhibit a specific gravityper filament of about 1.75, a denier per filament of 0.78, a mean singlefilament tensile strength of 350,000 psi, a mean single filament Youngsmodulus of 44 million psi, and an elongation of 0.78 percent.

Although the formation of the improved carbon filaments have beendescribed with preferred embodiments, it is to be understood thatvariations and modifications may be employed in the carbon filamentformation technique without departing from the concept of the presentinvention.

We claim:

1. An improved carbon filament comprising at least percent carbon byweight having an unusually highly developed microporous and fibrillarinternal structure substantially coextensive with the length of saidfila-,

ment capable of diverting a propagating crack during fracture therebyincreasing the amount of work required to break the filament asevidenced by a mean apparent fracture surface energy of at least 50joules per square meter.

2. An improved carbon filament according to claim 1 wherein saidfilament is present in a multifilament yarn.

3. An improved carbon filament according to claim 1 wherein saidfilament is present in a multifilament tow.

4. An improved carbon filament according to claim 1 wherein saidfilament comprises at least 95 percent carbon by weight.

5. An improved carbon filament according to claim 1 wherein saidfilament comprises at least 98 percent carbon by weight.

6. An improved carbon filament according to claim 1 which exhibits amean single filament Youngs modulus of about 20 to 50 million psi.

7. An improved carbon filament according to claim 1 wherein thecross-sectional configuration thereof is substantially round.

8. An improved carbon filament according to claim 1 which exhibits amean apparent fracture surface energy of at least 60 joules per squaremeter.

9. An improved carbon filament according to claim evidenced by a meanapparent fracture surface energy of at least 70 joules per square meter.

1. AN IMPROVED CARBON FILAMENT COMPRISING AT LEAST 90 PERCENT CARBON BY WEIGHT HAING AN UNSUALLY HIGHLY DEVELOPED MICROPOROUS AND FIBRILLAR INTERNAL STRUCTURE SUBSTANTIALLY COEXTENSIVE WITH THE LENGTH OF SAID FILAMENT CAPABLE OF DIVERTING A PROPAGATING CRACK DURING FRACTURE THEREBY INCREASING THE AMOUNT OF WORK REQUIRED TO BREAK THE FILAMENT AS EVIDENCED BY A MEAN APPARENT FRACTURE SURFACE ENERGY OF AT LEAST 50 JOULES PER SQUARE METER.
 2. An improved carbon filament according to claim 1 wherein said filament is present in a multifilament yarn.
 3. An improved carbon filament according to claim 1 wherein said filament is present in a multifilament tow.
 4. An improved carbon filament according to claim 1 wherein said filament comprises at least 95 percent carbon by weight.
 5. An improved carbon filament according to claim 1 wherein said filament comprises at least 98 percent carbon by weight.
 6. An improved carbon filament according to claim 1 which exhibits a mean single filament Young''s modulus of about 20 to 50 million psi.
 7. An improved carbon filament according to claim 1 wherein the cross-sectional configuration thereof is substantially round.
 8. An improved carbon filament according to claim 1 which exhibits a mean apparent fracture surface energy of at least 60 joules per square meter.
 9. An improved carbon filament according to claim 1 which exhibits a mean apparent fracture surface energy of at least 70 joules per square meter.
 10. An improved carbon filament comprising at least 95 percent carbon by weight which exhibits a mean single filament Young''s modulus of about 20 to 50 million psi and has an unusually highly developed microporous and fibrillar internal structure substantially coextensive with the length of said filament capable of diverting a propagating crack during fracture thereby increasing the amount of work required to break the filament as evidenced by a mean apparent fracture surface energy of at least 70 joules per square meter. 